Glasses generally break because tension acting on a surface flaw exceeds the engineering strength of the glass. Thus, glass products can be effectively strengthened if compression is introduced into the surface. The strengthening depends to a great extent on the flaw condition of the glass, which, in turn, depends upon how the glass has been handled through product manufacturing as well as subsequent user-handling stages. One practical means of glass strengthening is to immerse the product in a bath of molten alkali salt at a temperature well below the transition temperature of the glass. Recommended molten salt is potassium nitrate for a sodium ion-containing glass and sodium nitrate for a lithium ion-containing glass. The basic principle is to use salt bath alkali ions that are larger than those in the glass. An ion exchange between the salt alkali ions and the glass host alkali ions results in surface stuffing of ions. This may relax due to viscous flow if the ion exchange temperature is raised close to or beyond the glass transition range. The glass transition temperature, often denoted Tg in the literature, is also used in place of the annealing point temperature for which the viscosity of glass is stated to be 1013 Poise. According to the American Society for Testing Materials (“ASTM”) testing standard ASTM C336, stresses relax in a matter of minutes at the annealing point. A lower temperature standard viscosity reference point is the strain point at which the viscosity is 1014.5 poise. According to ASTM, stresses relax in a matter of hours at the strain point. These “chemical strengthening of glass” concepts relating to the viscosity of glass are described in the literature, for instance in U.S. Pat. Nos. 3,752,729 and 3,790,430, both by Mochel and assigned to Corning Glass Works (now Corning, Inc.). See also R. F. Bartholomew and H. M. Garfinkel, in Glass Science & Technology, editors D. R. Uhlmann and N. J. Kreidl, vol. 5, p. 217-270, Academic Press 1980; and A. K. Varshneya, Fundamentals of Inorganic Glasses, 1st edition, pages 339-344 and 445-448, Academic Press 1994.
As a test for glass strength, the ASTM recommends performing a modulus of rupture (“MOR”) test, see ASTM standard C158-84. One way to perform the MOR test is to load rectangular or circular cross-sectional beams in a 3-point or 4-point mode. The rectangular beam tests, in particular, tend to be edge condition-dependent and, hence, flaws on the sawed/ground edge are invariably the origin of the fracture. The MOR test is desensitized to edge conditions by performing concentric “ball-on-ring” or “ring-on-ring” (also called “concentric ring”) tests; see ASTM F-394.
Because the handling flaws are generally of the order of 10 to 50 microns, the performance of chemically strengthened glass products depends not only on the magnitude of the surface compression introduced but also on the “case depth,” which is the depth of the compression below the surface (depth at which the decreasing compression magnitude essentially reaches zero stress, changing over to a tension at larger depths). The ASTM standard C1422-99 thus classifies chemically strengthened flat glass products based on (i) the magnitude of surface compression and (ii) the magnitude of case depth. For protection against impacting projectiles, both should be as large as possible. Unfortunately, the ion exchange process, being atomic diffusion dependent, is extremely sluggish. Invading ion penetration only increases as the square root of time. It is also known, however, that the depth of surface compression does not increase as the square root of time. This is because, with time, some atomic adjustments occur to relieve the beneficial compression even at temperatures well below the strain point (see V. Tyagi and A. K. Varshneya, J. Non-Cryst. Sol. 238, p. 186-192 (1998); and M. D. Ingram et al., Glastech. Ber. Glass Sci. & Technol. 73(4) 89-104 (2000)). Thus, while minutes of immersion in a molten salt bath could be conducted at temperatures just below the strain point to avoid the relaxation of the beneficial surface compression, an ion exchange process requires hours or days of immersion to develop a sufficient case depth at temperatures well below the strain point of the glass.
For a given glass composition, the overall strengthening is a function of the type of invading ion (salt bath composition), bath temperature and immersion time. The need for proper ion exchange temperature and immersion time is demonstrated well for the chemical strengthening of a sodium aluminosilicate glass in a classic publication by Nordberg et al. J. Am. Ceram. Soc., vol. 47(5) p. 215-219 (1964) of which FIG. 9 is included herewith as FIG. 1 below for reference. Note that if “fast exchange” with good MOR were the intent, an optimized exchange would be 550° C., for 1 hour. However, if maximum MOR were the intent, for instance to fend against more severe handling, the ion exchange treatment would be at 450° C., continuing beyond 16 hours.
One commercially successful example of ion exchange-strengthened glass is aircraft windshield manufactured in the USA by PPG, Incorporated. With roughly 400 MPa (about 58,000 psi) of surface compression and a case depth of about 300 microns, such glass, when assembled as a multilayer laminate with polymers such as polyvinyl butyrate and polycarbonate, is designed to protect the aircraft from flying birds. In a quality control test such windshields exhibited full containment after impacting a 4-Lb standardized thawed chicken (or equivalently 4-Lb gel mass) flying from a gas-pressurized cannon at 400 knots. Several other applications of chemically strengthened glass do exist, such as for optical storage disk media, ophthalmic glass, and pharmaceutical containers, where the requirements of the compression magnitude and case depth are not so severe.
Aircraft windshield is a typical example of a high security application for multilayered glass and polymer laminate. The laminates are constructed to be thick enough such that the outboard plies of a tough polymer act to absorb the energy of an impacting projectile and the glass plies provide the resistance to flexure due to their high elastic moduli. After most of the energy of the projectile is absorbed by the “sacrificial” outer plies, the inboard plies of glass prevent the penetration due to their high resistance to fracture. One other function of the polymer interlayer is to retain fractured pieces of glass adhered to it instead of flying away as shards that otherwise could cause serious injury. The thickness of the composite not only provides energy absorption but also reduced flexure which, in turn, results in lesser tension on the “convexed” surface, hence, lesser probability of glass fracture. Unfortunately, the higher thicknesses of such composites can add greatly to the overall weight of the assembly which, in turn, may be detrimental to the performance of, say, a vehicle equipped with such glass laminate. The use of extremely high strength glass plies inboard reduces the need for high glass ply thickness, hence, forms a significantly superior design.
For architectural window applications that meet Florida Building Code, otherwise called the “hurricane code” (www.dca.state.fl.us/fhcd/fvc; Test Application Standards TAS 201, 202, and 203), glasses are often only heat-strengthened or fully tempered (“Kind-HS” or “Kind-FT” of ASTM C1048-97b) and laminated. The thermal tempering process, which takes only a few minutes to introduce surface compression, creates a case depth from each side that is roughly ⅕th of the glass thickness, thus about 1.2 mm deep for an approximately 6 mm thick conventional glass window. Unfortunately, the thermally tempered glasses are limited to achieving no more than about 100 MPa surface compression, which is rarely adequate for protection against high velocity flying debris in a Category 4 or stronger hurricane. Further, because the thermal tempering process necessarily raises the temperature of the glass to well above the glass transition temperature during the manufacturing process, some optical distortion of the glass due to viscous flow is unavoidable. Use of such windows in critical applications such as military armored vehicles, hence, is not recommended. Additionally, the high weight of glass windows in buildings designed to withstand hurricanes could present structural limitations.
In the practice of chemical strengthening of glass, it is well known that the soda-lime-silicate glasses, which generally form the basis of most architectural and windshield applications, can only be strengthened to a depth of a few microns using immersion in potassium nitrate bath. Use of higher temperatures relaxes the beneficial compression and the use of lower bath temperatures requires days of immersion substantially increasing the costs of manufacturing. A publication by Nordberg et. al. from Corning Glass Works (J. Amer. Ceram. Soc., vol. 47, p. 216-219, 1964) discusses the strengthening of a 9 mol % lithium oxide-containing aluminosilicate glass immersed in a molten sodium nitrate salt bath at about 400° C., for 4 hours. With alumina increasing from 14 to 40%, tumble-abraded strengths increased from about 35 Kpsi (1000 psi) to as much as 100 Kpsi. Case depths obtained were 0.010 inch (about 250 microns) to 0.020 inch (about 500 microns).
Garfinkel and King (J. Amer. Ceram. Soc., vol. 53, p. 686, 1970) disclose strengthening a 5.15 wt. % lithium oxide-containing aluminosilicate glass immersed in sodium nitrate salt bath at 404° C., for 4 hours, to achieve a case depth of about 210 microns and an MOR of about 97 Kpsi. U.S. Pat. No. 3,790,430 by Mochel describes the chemical strengthening of an 18 mol % lithium oxide-containing aluminosilicate glass immersed in a molten sodium nitrate salt bath at about 400° C., for 4 hours. Strengths of 106,000 psi were claimed to have been achieved. However, no measurements of case depth were reported.
The strengthening of sodium aluminosilicate glasses by immersion in potassium nitrate salt bath is described in U.S. Pat. No. 4,119,760 by Rinehart and assigned to Pittsburgh Plate Glass Company, now PPG, Incorporated. The compositions are claimed to have been suitable for forming using the conventional “updraw” or “float” processes for making flat glasses. Short term exchange such as 900° F. (482° C.) and 4 hours in a potassium nitrate bath was said to produce MOR levels of about 90 Kpsi with a case depth of only 50 microns. The MOR degraded rapidly to 49 Kpsi (Table IV) as the exchange temperature was increased to 1050° F. (565° C.) for times as little as ½ hour.
It is known that achieving a case depth of the order of 150 microns or more requires lithium to sodium ion exchange (lithium ions in the glass exchanged by sodium ions in a molten salt bath in which the glass is immersed). Therefore, for strengthening lithium-containing glass, the conventional salt bath contains almost all sodium salt, such as sodium nitrate. U.S. Pat. No. 3,357,876 by Rinehart and assigned to Pittsburgh Plate Glass Company, discusses strengthening lithia-soda-alumina-phosphorus pentoxide-silica glasses containing 1 to 25 wt % phosphorus pentoxide, by immersion in salt baths containing sodium nitrate or potassium nitrate and in serially conducted exchange experiments where each successive treatment is conducted using an alkali metal which is larger in ionic size than that employed in the prior experiment. This patent disclosure requires phosphorus pentoxide (P2O5) and Na2O in the base glass.
U.S. Pat. No. 3,433,611 by Saunders et al. and assigned to PPG Industries, Inc., also describes chemically strengthening lithia-soda-aluminosilicate glass that includes phosphorus pentoxide. This patent discloses using a mixed salt bath having a ratio of potassium nitrate salt to sodium nitrate salt in a range of 2:1 to 50:1. The patent discloses that the stress profiles of the lithia-soda-alumina-phosphorus pentoxide-silica glasses obtained by immersion in an 8:1 potassium nitrate to sodium nitrate bath at 875° F. (468° C.) for 60 minutes show surface compression of about 96 Kpsi dropping steeply to about 35,000 at a depth of 20 microns and subsequently falling to zero around 220 microns depth (Example 11, paragraph 6; FIG. 3).
U.S. Pat. No. 3,410,673 by Marusak and assigned to Corning Glass Works (now Corning Inc.) discloses a method of chemically strengthening lithium aluminosilicate glass containing lithium and sodium ions. The glass may be subjected to immersion in multiple salt baths, for example, immersion in a bath of sodium salt, followed by immersion in a potassium salt bath. The purpose indicated is to first exchange lithium in the glass by sodium from the bath and, subsequently, lithium and/or sodium ions in the surface of glass by potassium ions from the bath. The patent also discloses using a salt bath comprising a 50:50 mixture of sodium and potassium salt to strengthen a glass which originally contained both lithium and sodium.
To the inventor's knowledge none of the above-mentioned glasses or processes described in numerous patents has been commercially utilized in the application of resisting fracture upon impact of high velocity projectiles (e.g., bullets or high wind velocity hurricanes). It has been observed that a primary difficulty in some of the glass compositions is the presence of large amounts of phosphorus pentoxide which, due to its extreme tendency for volatilization during typical glass-melting processes such as those needed in float glass or sheet updraw production, is not a desirable constituent. U.S. Pat. No. 4,156,755, by Rinehart and assigned to PPG Industries, Inc., discloses glasses having 59-63% SiO2, 10-13% Na2O, 4-5.5% Li2O, 15-23% Al2O3 and 2 to 5% ZrO2. Rinehart's glass composition avoided using B2O3 and P2O5 in view of their volatility but employed a high amount of Na2O. In addition, Rinehart chemically strengthened the glass by immersion in pure sodium nitrate bath at 705° F., for 22 hours (or 750° F. for 4 hours). An MOR of about 45 Kpsi with a case depth of about 190 microns were reported.
It is recognized that, whereas about 400 MPa surface compression and about 300 micron case depth in 2-3 sheets of laminated glass are adequate for aircraft windshields, these values are inadequate for providing resistance of a glass transparency against bullets or flying debris from high wind velocity hurricanes. Both the magnitudes of surface compression as well as the case depth need to be increased for glass products under such extreme conditions. The problem lies in the facts that, in case of potassium exchange for sodium in sodium-containing glasses, although the surface compression generated is large, the case depth developed is generally less than 100 microns. As shown above, potassium ions from a potassium-containing salt bath exchanging for lithium ions in glass is even slower. Efforts to increase the case depth by increasing the temperature to affect a faster exchange or time to increase invading ion penetration generally result in relaxation of the beneficial surface compression.
A published U.S. patent application 20050090377 by Shelestak et al., assigned to PPG Industries, Inc., discloses using high strain point lithium aluminosilicate glasses and a sodium salt bath or predominantly sodium, mixed Na/K salt bath, in an attempt to achieve deeper surface compression and higher MOR. The authors disclose lithium aluminosilicate glasses having a composition comprising (in weight %): Li2O in a range of 3 to 9%, Na2O+K2O in an amount of less than 3.5% and Al2O3 in an amount ranging from 7% to 30%. The molten salt bath disclosed in the '377 patent application preferably uses at least 50% sodium salt (generally sodium nitrate) and, in particular 100% sodium salt, to enable sodium exchange. This reference discloses case depths of, for example, 11.5 to 17.4 mils (about 300 to 450 microns).
In addition, Sandia National Laboratories have proposed strengthening a high annealing point lithium aluminosilicate glass (Schott ROBAX® glass composition) using a pure potassium nitrate salt bath (see S. Jill Glass et al., paper no. GOMD-S2-29-2004, American Ceramic Society, Glass & Optical Materials Div. meeting, Cocoa Beach Fla., Nov. 7-12, 2004). This purportedly produced, for example, high surface compression of 850 MPa, but low case depths of only 40 microns that took days of immersion to achieve (shown in Table 1 below in the column for Schott).
Similarly, U.S. Pat. No. 6,814,453 by Miwa and Kanai and assigned to Nippon Electric Glass Co., Ltd., describes strengthening high annealing point lithium aluminosilicate glass in a bath of potassium nitrate at 500° C. for about 6 hours to develop a strengthened substrate to prevent projector lamp fragments from flying forward after sudden failure. The depth of compression stated is only about 20 to 30 microns. The Miwa et. al. patent essentially confirms the results of Sandia National Laboratories, that exchange of lithium ion in glass for potassium ion from a salt bath is a very slow process.
Despite all of the work in the area of chemical strengthening of glass, a need remains for a glass having high strength to resist fracture upon flexing from impact of high velocity projectiles. Such a glass will need to be formable by commercially available glass-forming processes with a temperature-time schedule that enables efficient and cost-effective chemical strengthening.
Many additional features, advantages and a fuller understanding of the invention may be understood from the following drawings and detailed description of the invention. Specific embodiments, figures and examples should not be construed as necessary limitations of the broad invention as defined in the claims and their equivalents.